Alkene hydrocarbons are primarily produced by pyrolysis of naphtha obtained from a petroleum refining process. They are important raw materials that form the basis of the current petrochemical industry. However, they are generally produced along with alkane hydrocarbons such as ethane and propane. Thus, alkene hydrocarbons/alkane hydrocarbons separation technology is of significant importance in the related industry.
Currently, the traditional distillation process is used mostly for the separation of an alkene/alkane mixture such as ethylene/ethane or propylene/propane. The separation of such a mixture, however, requires the investment of large-scale equipment and high-energy cost because alkene and alkane have similarities in molecular size and physical properties such as relative volatility.
In the distillation process used hitherto, for example, a distillation column having about 120–160 trays should be operated at a temperature of −30° C. and a high pressure of about 20 atm for separation of an ethylene and ethane mixture. For separation of a propylene and propane mixture, a distillation column having about 180–200 trays should be operated at a temperature of −30° C. and a pressure of about several atms in the reflux ratio of 10 or more. As such, there has been a continuous need for the development of a new separation process that can replace the prior distillation process, which requires the investment of large-scale equipment and high-energy cost.
A separation process that could be considered as a replacement for said prior distillation process is one that uses a separation membrane. Separation membrane technology has progressed remarkably over the past few decades in the field of separating gas mixtures, for example, the separation of nitrogen/oxygen, nitrogen/carbon dioxide and nitrogen/methane, etc.
However, the satisfactory separation of alkene/alkane mixtures cannot be accomplished by using traditional gas separation membranes because alkene and alkane are very similar in terms of their molecular size and physical properties. A facilitated transport membrane based on a different concept from the traditional gas separation membranes is considered to be a separation membrane that can achieve excellent separation performance for alkene/alkane mixtures.
The separation of mixtures in a separation process using a separation membrane is achieved by the difference in permeance between the individual components constituting the mixtures. Most materials of a separation membrane have many limitations on their application because of an inverse correlation between permeance and selectivity. However, the concurrent increase of permeance and selectivity is made possible by applying a facilitated transport phenomenon. Consequently, the scope of their application can be considerably increased. If a carrier capable of selectively and reversibly reacting with a specific component of a mixture is present in a separation membrane, mass transport is facilitated by additional material transport generated from a reversible reaction of a carrier and a specific component. Therefore, overall mass transport can be indicated by Fick's law and the sum of material transport caused by a carrier. This phenomenon is referred to as facilitated transport.
A supported liquid membrane is an example of a membrane prepared by applying the concept of facilitated transport. The supported liquid membrane is prepared by filling a porous thin layer with a solution that is obtained by dissolving a carrier capable of facilitating mass transport in a solvent such as water, etc. Such a supported liquid membrane has somewhat succeeded.
Steigelmann and Hughes, for example, prepare a supported liquid membrane in which the selectivity of ethylene/ethane is about 400–700 and the permeance of ethylene is 60 GPU [1 GPU=1×10−6 cm3 (STP)/cm2·sec·cmHg], which are satisfactory performance results for permeance separation (see U.S. Pat. Nos. 3,758,603 and 3,758,605). However, the supported liquid membrane exhibits the facilitated transport phenomenon only under wet conditions. There is an inherent problem in that its initial permeance separation performance cannot be maintained for an extended period of time due to solvent loss. Thereby, the separation performance is decreased with time.
In order to solve the problem, Kimura, etc., suggest a method that enables facilitated transport by substituting a suitable ion in an ion-exchange resin (see U.S. Pat. No. 4,318,714). This ion-exchange resin membrane also has a drawback, however, in that the facilitated transport phenomenon is exhibited only under wet conditions, similar to the supported liquid membrane.
Ho suggests another method for the preparation of a complex by using water-soluble glassy polymer such as polyvinyl alcohol (see U.S. Pat. Nos. 5,015,268 and 5,062,866). However, the method also has a drawback in that satisfactory results are obtained only when feed gas is saturated with water vapor by passing the feed gas through water or when a membrane is swelled with ethylene glycol or water.
In all the instances described above, the separation membrane must be kept in wet conditions that enable the membrane to contain water or other similar solvents. When a dry hydrocarbon gas mixture—for example, an alkene/alkane mixture free of a solvent such as water—is separated by using the membrane, solvent loss is unavoidable with time. Therefore, a method is necessary for periodically feeding a solvent to a separation membrane in order to continuously sustain the wet condition of the separation membrane. It is, however, rarely possible for the method to be applied to a practical process, and the membrane is not stable.
Kraus, etc., develop a facilitated transport membrane by using another method (see U.S. Pat. No. 4,614,524). According to the patent, a transition metal ion is substituted in an ion-exchange membrane such as Nafion, wherein the membrane is plasticized with glycerol, etc. The membrane could not be utilized, however, in that its selectivity of ethylene/ethane is as low as about 10 when dry feed is used. The membrane also has no selectivity when a plasticizer is not used. Furthermore, the plasticizer is lost with time.
In view that a usual polymer separation membrane cannot separate an alkene/alkane mixture having similar molecular size and physical properties, as described above, use of a facilitated transport membrane capable of selectively separating only alkane is necessary. In conventional facilitated transport membranes, however, the activity of a carrier is maintained by using one of the following methods: filling a solution containing a carrier into the porous membrane; adding a volatile plasticizer; or saturating a feed gas with water vapor. Such a membrane cannot be utilized due to the problem of declining stability since components constituting the membrane are lost with time. There is also the problem of later having to remove solvents such as water, etc., which are periodically added in order to sustain activity, from the separated product.
Therefore, there is a need for the development of a separate membrane that can replace the prior distillation process requiring the investment of large-scale equipment and high-energy cost in the separation of an alkene/alkane mixture. The separation membrane in this regard would not contain volatile components and would have high selectivity and permeance so as to maintain the activity even under long-term dry operating conditions.